The invention relates to a device for the cyclic rearrangement of a pile of rectangular or square sheets, or a so-called "picture-changer".
Picture changers are known from U.S. Pat. Nos. 4,238,898, 4,238,899, 4,241,528, 4,241,529, 4,245,417, 4,259,802 and 4,376,348. These specifications are all based on the principle that a pile of pictures, especially photographic prints, is held by two frame parts that are movable relative to one another, one of which may have a viewing window. During each complete cycle of movement of the frame parts, that is, pulling them fully away from each other and sliding them fully back together again, one picture is removed from one end of the pile and returned to the other end of the pile again. The picture changers have the following components for this:
A feeding means feeds pictures to a separating means; for separating means detaches an individual picture from the pile; a retaining means holds the individual picture separated from the pile in one of the frame parts whilst the remainder of the pile is held in the other frame part; a guide means guides the separated individual picture such that it goes onto the other end of the remainder of the pile.
Photographic prints currently have a thickness of approximately 0.25 mm and have a tendency to cling to one another with considerable force as a result of static charging of their plastic coatings or when their surface is still damp following processing; this happens particularly in the picture changers already mentioned, where the pile of prints is pressed against a viewing window and static charges are generated by the change-over operation. Adhesives forces of up to 300 g may occur and these have to be overcome by what amounts to a shearing action when separating the sheets. In order to be able to undertake trouble-free separation of the sheets under these conditions in a picture changer for photographic prints, it is therefore necessary to generate a correspondingly high shearing force between the sheet that is to be separated and the sheet of the remainder of the pile that immediately follows it in the pile.
Photograph changers are mass-produced parts, chiefly assembled from injection-moulded plastic parts, and are subject to relatively large manufacturing tolerances. In addition, they are subjected to considerable mechanical stress through handling and to considerable thermal stress, for instance from exposure to sunlight, which in use may lead to deformation of the components which are usually of thin-walled construction for reasons of cost. Added to this is the fact that photographic prints are not only, as already mentioned, very thin, but also warp and bow on all sides under the influence of humidity and fluctuations in temperature.